Method of chromizing ferrous metal



0a. 7, 195 SAMUEL 2, 55,332

METHOD OF CHROMIZING FERROUS METAL Filed Sept. 7, 1954 (f/BOM/Z IAG COM/ 057270 I v. INVENTOR 1 George A- Samua/ United States Patent METHOD OF CHROMIZING FERROUS METAL George A. Samuel, Philadelphia, Pa., assignor to Metal Dilfusions, Inc., Philadelphia, Pa., a corporation of New Jersey Application September 7, 1954, Serial No. 454,396

4 Claims. (Cl. 117--107) The present invention relates to chromizing of metal parts, particularly ferrous metal parts such as iron and steel.

A purpose of the invention is to obtain greater penetration of chromium in a given time at a given temperature.

A further purpose is to minimize the extent of side reactions between a halogen catalyst, particularly fluorine, and iron, and concentrate the reaction as far as possible to diffuse chromium.

A further purpose is to accomplish chromizing with less pickling and etching of the surface of the article being chromized and obtain a better final surface.

A further purpose is to obtain intimate adherence between the chromizing compound and the article by forming an adhesive coatingof the chromizing compound on the article which has the inherent property of becoming friable at elevated temperature. p

A further. purpose isto apply the chromizing ingredients in a watery dispersion which has a non-carbonaceous low temperature bond which will release at elevated temperature.

A further purpose is to apply the coating selectively in different thicknesses to obtain diffused layers of different depths.

Further purposes appear in the specification and in the claims.

In the drawings,

Figure 1 shows in axial section one embodiment of an article to which the coating of the invention may be applied.

Figure 2 is a central vertical diagrammatic section of apparatus which may be used in practicing the invention. 1

.Chromizing has been heretofore usually accomplished by using a loose powdered chromizing compound, cornmonly including ferrochrome or other source of chromium, and ammonium chloride or iodide. In this procedure when the compound is heated to chromizing temperature, the ammonium halide breaks down, liberating hydrogen, nitrogen, and halogen, and the halogen attacks both thesource of chromium and also the surface of the ferrous metal article, forming a considerable amount of ferrous halide which ties up catalyst. Furthermore the halogen has a tendency to pickle or etch the surface of the article, and often causes substantial deterioration on finished metal surfaces.

. Due to the distance which the chromium must travel before diffusion begins, from the compound to the article, the process is relatively slow acting and diffusion rates are not exceptional. The active gases are diluted .to a considerable extent with ferrous halide vapor.

The present invention is concerned with overcoming these difliculties and particularly obtaining greater penetration in a given time at a particular temperature, desirably by minimizing the extent to which undesired ferrous halide is produced.

Furthermore by the present invention, the resulting surface is improved and there is markedly less pickling and etching.

In accordance with the present invention furthermore, the chromizing composition is preferably applied as an initially adhering coating on the article being chromized, thus avoiding the need for extensive distances of travel by chromizing gases, and also eliminating or minimizing the labor incident to packing chromizing compound around the articles.

In accordance with the invention, the catalyst used is a compound of chromium and fluorine. While any one of the fluorides of chromium may be employed, it is decidedly preferable for several reasons to use a complex ammonium fluoride. One advantage is that this compound breaks down, liberating hydrogen and nitrogen, sweeping air out of the container and protecting against oxygen contamination during the process.

Another advantage is that when the compound breaks down, the chromium or chromium halide is available in nascent form, where the reactivity is correspondingly increased.'

While chromium fluoride or a complex ammonium chromium fluoride can be used from any suitable source of such materials, it is preferable to produce the catalyst by reaction in making the compound. I desirably react ammonium bifluoride in water solution with a source of chromium, which may for example be either metallic chromium or ferrochrome in finely divided form, and a violent reaction takes place with evolution of heat and gas, forming a complex ammonium chromium fluoride. As I desirably have a considerable excess of chromium present, there is an excess of chromium which forms a dispersion or slurry. I desirably also include an inert powdered bodying ingredient which will prevent close compacting of the chromium particles and sintering at elevated temperatures. The inert bodying material may be finely divided alumina, silica, or clay such as kaolin. Any other inert bodying powder may be used.

This slurry or dispersion has some remarkably advantageous properties from the standpoint of the present invention. When it is coated by brushing, spraying, dip ping or otherwise on the ferrous metal article and allowed to dry, it has a bonding or adhesive action which forms a firm adhering coating and will even permit rough handling of the work in packing in the box. This bond is not a carbonaceous bond like the usual adhesives, thus avoiding the tendency to carburize the work. This is unusual, as it is very difficult to find a non-carbonaceous adhesive available for this purpose.

An important aspect of the invention is that after the chromizing takes place and the complex ammonium fluoride is broken down, the adhesive action largely disappears and the coating becomes friable and readily flakes.

Ammonium bifluor Ferrochromium (preferably the 65% by weight chromium grade) 67.5

Inert bodying ingredient, preferably alumina powder 17 Water Balance In making up this composition it is preferable to proceed as follows:

10 parts by weight of ammonium bifluoride is dissolved in 90 parts by weight of water. A mixture of percent by weight of ferrochromium powder and 20 percent by weight of alumina is made up, and 6 parts latented Oct. 7, 19 58 by weight of this mixture is added by stirring into one part by weight of the solution of ammonium bifiuoride. A violent reaction occurs, generating heat and evolving gas, and this may continue for about two hours. When the resulting slurry is cold, it is ready for use. If preferred, itcan be allowed to dry, and then stirred up with water to the original volume. The dry'powder can be shipped and mixed up with water at a distant point. 'I'heapplication to the ferrous metal work can be by dipping, brushing, spraying or any other coating technique.

There is, of course, a great excess of chromium in the above mixture and thequantity of ammonium bifiuoride is not critical. Good results can be obtained by using as little as 0.1 percent by weight of ammonium bifiuoride or as much as percent by weight of ammonium bifluoride, adjusting the quantity of water if required to obtain a suitable slurry.

The coating bonds at low temperatures on the ferrous metal articles when it dries, and the coated articles can be stored for considerable periods of time before chromizing if desired.

The chromizing will be carried out in a closed box, preferably at a temperature of 1650 to 1950 degrees F., and preferably about 1750 degrees F., suitably for a time in excess of three hours and usually in excess of five hours, and desirably for about eighteen hours. The final chromized coating is bright and the deposit of compound scales off readily. One of the advantages of this technique is that the coating is very intimately associated with the product.

The results obtained have been excellent and the improvement in penetration in a given time over the box method using ammonium bifiuoride which has not previously reacted with the chromium is from 50 to 75 percent. The film of the coating before chromizing is usually of the order of 0.010 inch thick. Chromizing deposits of 0.002 inch thick free from porosity are readily obtained.

The particles should preferably be through 100 mesh per linear inch. In the broadest aspects of the invention, I use a coating which bonds with a non-carbonaceous bond, and later becomes friable.

In some cases it is desirable to apply selected coatings of the dried dispersion as above described of various thicknesses. Thus in the drawing I show a steel valve having a seat surface 21 and a stem 22. A single coating 23 extends over the entire valve and dries before a second coating 24 has been applied extending over the seat surface so as to cause diffusion to extra thickness on the seat surface. It will further be evident that stopoif techniques can be used in dipping the article in the watery dispersion, as well as in spraying or brushing the dispersion, as, for example, by applying grease to areas which are not intended to receive the chromizing coating.

In operation the articles to be chromized are simply dipped in the chromizing coating and the coating is allowed to dry. The articles are then placed in the chromizing .box, either being supported by fixtures or by any suitable inert packing material, or if their shape permits it, being stacked or nested one upon the other in the box. This latter procedure is particularly suitable for articles which will form a pack such as sheets or which will nest such as stampings. The coating prevents objectionable contact between one article and the next.

The articles coated with the dried chromizing coating according to'the present invention are suitably formed into a pack or pile26 which may "fill the bottom 27 of a box 28 of suitable heat-resisting alloy. A bell top 4 30 covers the box, preferably making a double seal, by a flange 31 into a channel 32 on the upper edge of the bottom, and by the bottom edge 33 into a channel 34 from the box bottom. The channels may be filled with powdered glass or a salt which will form a molten seal at operating temperature.

The fluorine compound dissociates as the box is heated and forms a reducing atmosphere sweeping out and excluding air, while at the same time the fluorine acts as a carrier of chromium to diffuse into the steel.

The time required will depend of course upon the thickness desired and the temperature, the time usually being from 1 to 24hours at a temperature of from 1650 to 1950 degrees F.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need Will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the process and composition, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The method of chromizing, which comprises reacting ammonium .bifluoride with chromium in watery solution to form a complex ammonium chromium fluoride, with an excess of chromium, coating the watery dispersion thus formed on a ferrous metal article to be chromized, and heating the article with exclusion of air at a temperature of from 1650 to 1950 degrees F. for a time of at least three hours.

2. The method of chromizing, which comprises coating a ferrous metal article with a chromizing composition including chromium and a complex fluoride of ammonium and chromium, in a watery dispersion, drying the composition and by the drying adhesively bonding the composition to the surface of the ferrous metal article, and subjecting the ferrousmetal article thus coated, with exclusion of air, to a temperature of from 1650 to 1950 degrees F. for a time of at least three hours, while concurrently diffusing' chromium into the article and deterring the adhesive action of the composition so as to render the composition friable for ready removal from the article.

3. The method of chromizing a ferrous metal article, which comprises coating saidferrous metal article with a chromizing composition including chromium and a complex fluoride of ammonium and chromium, in watery dispersion, which dispersion has the property of setting on drying and is friable on being heated at chromizing temperature, drying said coating on the ferrous metal -article,.and heating said article thus coated at chromizing temperature, with exclusion of air, until the article is chromized.

4. The method of chromizing a ferrous metal article as set forth in claim 3, wherein on heating said coating at the chromizing temperature, a protective gaseous atmosphere substantially inert to etching the surface of .said ferrous article is generated by the coating in the environment of the article and accordingly affords chrornium for chromizing the article.

References Cited in the file of this patent UNITED STATES PATENTS 1,853,369 Marshall Apr. 12, 1932 2,046,638 Lauenstein July 7, 1936 2,157,594 Cooper May 9, 1939 2,536,774 Samuel Jan. 2, 1951 2,657,127 .Sindeban'd Oct. 27, 1953 

1. THE METHOD OF CHROMIZING, WHICH COMPRISES REACTING AMMONIUM BIFLUORIDE WITH CHROMIUM IN WATERY SOLUTION TO FORM A COMPLEX AMMONIUM CHROMIUM FLUORIDE, WITH AN EXCESS OF CHROMIUM, COATING THE WATERY DISPERSION THUS FORMED ON A FERROUS METAL ARTICLE TO BE CHROMIZED, AND HEATING THE ARTICLE WITH EXCLUSION OF AIR AT A TEMPERATURE OF FROM 1650 TO 1950 DEGREES F. FOR A TIME OF AT LEAST THREE HOURS. 